feat: Add update_theme functionality with UI support

Backend:
- Add PUT /api/themes/:id endpoint for updating existing themes

Store:
- Add updateTheme method to theme store

MCP:
- Add update_theme tool to modify name, description, or save current variables

UI:
- Add edit button to ThemeListItem (custom themes only)
- Add edit modal with name and description fields
- Support editing from both desktop sidebar and mobile dropdown

frontend/src/pages/ThemesPage.vue

@@ -16,6 +16,10 @@ const showNewThemeModal = ref(false)
 const showCloneModal = ref(false)
 const cloneSourceId = ref<string | null>(null)
 const cloneName = ref('')
+const showEditModal = ref(false)
+const editingTheme = ref<any>(null)
+const editName = ref('')
+const editDescription = ref('')
 const fileInput = ref<HTMLInputElement | null>(null)
 
 const currentCategoryVariables = computed(() => {
@@ -58,6 +62,26 @@ async function confirmClone() {
   }
 }
 
+function handleEditTheme(theme: any) {
+  editingTheme.value = theme
+  editName.value = theme.name
+  editDescription.value = theme.description || ''
+  showEditModal.value = true
+}
+
+async function confirmEdit() {
+  if (editingTheme.value && editName.value.trim()) {
+    await store.updateTheme(editingTheme.value.id, {
+      name: editName.value.trim(),
+      description: editDescription.value.trim() || undefined
+    })
+    showEditModal.value = false
+    editingTheme.value = null
+    editName.value = ''
+    editDescription.value = ''
+  }
+}
+
 function handleSetDefault(id: string) {
   store.setDefaultTheme(id)
 }
@@ -171,6 +195,7 @@ onMounted(() => {
                   @select="(t) => { handleSelectTheme(t); mobileDropdownOpen = false }"
                   @clone="handleCloneTheme"
                   @setDefault="handleSetDefault"
+                  @edit="handleEditTheme"
                 />
               </div>
               <div v-if="store.userThemes.length > 0" class="dropdown-section">
@@ -184,6 +209,7 @@ onMounted(() => {
                   @delete="handleDeleteTheme"
                   @clone="handleCloneTheme"
                   @setDefault="handleSetDefault"
+                  @edit="handleEditTheme"
                 />
               </div>
             </div>
@@ -208,6 +234,7 @@ onMounted(() => {
             @delete="handleDeleteTheme"
             @clone="handleCloneTheme"
             @setDefault="handleSetDefault"
+            @edit="handleEditTheme"
           />
         </section>
 
@@ -222,6 +249,7 @@ onMounted(() => {
             @delete="handleDeleteTheme"
             @clone="handleCloneTheme"
             @setDefault="handleSetDefault"
+            @edit="handleEditTheme"
           />
         </section>
       </div>
@@ -366,6 +394,33 @@ onMounted(() => {
         </div>
       </div>
     </div>
+
+    <!-- Edit Modal -->
+    <div v-if="showEditModal" class="modal-overlay" @click="showEditModal = false">
+      <div class="modal" @click.stop>
+        <h3>Edit Theme</h3>
+        <div class="form-group">
+          <label>Name</label>
+          <input
+            v-model="editName"
+            type="text"
+            placeholder="Theme name"
+          />
+        </div>
+        <div class="form-group">
+          <label>Description</label>
+          <textarea
+            v-model="editDescription"
+            placeholder="Theme description (optional)"
+            rows="3"
+          ></textarea>
+        </div>
+        <div class="modal-actions">
+          <button class="btn-secondary" @click="showEditModal = false">Cancel</button>
+          <button class="btn-primary" @click="confirmEdit">Save</button>
+        </div>
+      </div>
+    </div>
   </div>
 </template>
 
@@ -820,6 +875,35 @@ onMounted(() => {
   border-color: var(--accent);
 }
 
+.form-group {
+  margin-bottom: 1rem;
+}
+
+.form-group label {
+  display: block;
+  font-size: 0.8rem;
+  font-weight: 500;
+  color: var(--text-secondary);
+  margin-bottom: 0.375rem;
+}
+
+.modal textarea {
+  width: 100%;
+  padding: 0.75rem 1rem;
+  background: var(--bg-primary);
+  border: 1px solid var(--border-color);
+  border-radius: 8px;
+  color: var(--text-primary);
+  font-size: 0.95rem;
+  font-family: inherit;
+  resize: vertical;
+}
+
+.modal textarea:focus {
+  outline: none;
+  border-color: var(--accent);
+}
+
 .modal-actions {
   display: flex;
   gap: 0.75rem;